1. Field of the Invention
This invention relates generally to wide angle optical imaging systems, including such systems that are suitable for use on motor vehicles.
2. Description of the Related Art
Manufacturers of motor vehicles are increasingly equipping their vehicles with imaging systems. For example, manufacturers are installing imaging systems at the rear of the vehicle to assist drivers when the vehicle is backing up. To increase the visibility for drivers and potentially eliminate blind spots altogether, these imaging systems preferably have a very wide field of view. Fields of view between 160-200 degrees (at least along the horizontal direction) are desirable to fully capture the scene at the rear of a vehicle. In addition to rear-looking systems, vehicular mounted imaging systems can also be used for other purposes, for example to detect lane markers or other guidance markings, to allow the driver to view directions other than rear-looking and possibly even to view the driver, the passengers or other interior areas of the vehicle.
Regardless of the application, it is usually desirable for the wide-angle vehicular-mounted wide angle lens system to be easy to manufacture, have a fast F# (i.e., low numerical aperture), and have a short total track length (i.e., the physical distance from the lens surface closest to the object to the image plane). Previous designs for optical systems have satisfied some of these requirements, but not all of them. Some designs require only a few easy-to-manufacture lens elements, but require long total track lengths. Others shorten the total track length but require many lens elements and often suffer from illumination fall-off or severe distortion. Examples of wide angle optical systems are described in U.S. Pat. Nos. 4,525,038, 6,844,991, 7,023,628, 7,173,776, and U.S. Patent Publ. No. 2006/0274433A1.
Thus, there is a need for a high quality wide angle imaging system, where the wide angle optical system has a short total track length, a fast F#, and only a few lens elements, preferably without any cemented lens elements.